Fix MSVS builds for unit test changes

Update the Visual Studio builds to support the new monolithic unit
test binary.
Includes minor semi-cosmetic refactoring of solution.mk, as the
%vpx.vcproj match is no longer appropriate given the test_libvpx
target.
Change-Id: I29e6e07c39e72b54a4b3eaca5b9b7877ef3fb134